1. Field of the Invention
The present invention relates to a display system of an imaging camera for use in an imaging apparatus such as a video camera or a still camera and, more particularly, to a display system of a camera having a tracking apparatus for detecting a moving amount of an object to be photographed in accordance with an output from an imaging device to automatically track the object to be photographed.
2. Description of the Related Art
In recent years, as a photometer for measuring the luminance of (performing photometry for) an object to be photographed for exposure control of a photographic instrument such as a still camera (to be abbreviated to a camera hereinafter) as an imaging apparatus, a photometer arranged to perform photometry for a region of an object to be photographed desired by a photographer is known. For example, a photometer as disclosed in Published Unexamined Japanese Patent Application No. 53-19027 can perform photometric operations for a central portion of a picture to be taken a plurality of times while changing framing of a camera, thereby performing photometry of a plurality of portions of an object to be photographed and simultaneously displaying the photometered values.
As a photometer of this type, an apparatus using so-called spot photometry as disclosed in Published Examined Japanese Patent Application No. 61-37566 is known. The spot photometry is a method of measuring the luminance of an object to be photographed by performing photometry for a considerably narrow central region of a picture of an imaging instrument such as a camera. When a photometer using spot photometry is mounted on, e.g., a still camera, however, framing of the camera is determined such that an object to be photographed is located at the center of a picture, and photometry and exposure control are performed for the object to be photographed. Thereafter, framing is changed for photographing, and then photographing is performed. Therefore, if an object to be photographed moves from the center of the picture and its luminance is changed after photometry and before photographing, exposure cannot be correctly performed.
In contrast to this, an automatic tracking apparatus does not store a photometric value before photographing but automatically tracks an object to be photographed so that a region to be subjected to spot photometry constantly tracks the object to be photographed, thereby performing photometry and exposure control immediately before photographing.
For example, an automatic tracking apparatus as disclosed in Published Examined Japanese Patent Application No. 59-32742 detects a moving amount of an object to be photographed in accordance with an output from an imaging device so as to allow a sensing mechanism such as an AF (automatic focusing mechanism) or an AE (automatic exposure mechanism) to track the object to be photographed.
In addition, an automatic tracking apparatus disclosed in Published Examined Japanese Patent Application No. 60-28475 calculates a correlation coefficient between a picture obtained by performing analog/digital conversion for an image signal supplied from an imaging means and a reference picture stored in advance and detects a maximum correlation point, and detects a curvature near the peak value of the correlation coefficient while tracking a predetermined target or scene present in the field of the imaging means, and calculates the size of a tracking window which maximizes the detected curvature.
An automatic tracking apparatus as disclosed in Published Unexamined Japanese Patent Application No. 64-1921 is also known. This apparatus initially determines framing of a camera so that an object to be photographed is located at the center of a picture, and measures a luminance distribution of the object to be photographed. If the object to be photographed moves, the apparatus determines a moving amount and a moving direction of the object to be photographed in accordance with a change in luminance distribution to automatically move a photometric region, thereby allowing a central portion of the photometric region to track the object to be photographed.
An automatic tracking apparatus for use in a video camera disclosed in Published Unexamined Japanese Patent Application No. 60-253887 detects a moving amount of an object to be photographed by using a phenomenon in which a color signal extracted from a video signal corresponding to the object to be photographed is shifted in accordance with the movement of the object to be photographed, and performs automatic tracking by using data of the detected moving amount.
An automatic tracking apparatus disclosed in Published Unexamined Japanese Patent Application No. 61-12177 registers color information of an object to be tracked concerning the size of a tracking field which is manually set, and detects a relative moving amount on the basis of the feature of the registered object to be tracked.
Of these conventional techniques, according to the apparatus disclosed in Published Unexamined Japanese Patent Disclosure No. 53-19027, a photographer can simultaneously check photometric values of a plurality of portions but cannot check the exact positions of these portions. Therefore, a photographer must remember positions of an object to be photographed which are subjected to photometry.
In the automatic tracking apparatus disclosed in Published Examined Japanese Patent Application No. 59-32742, after a luminance signal of each pixel is subjected to analog/digital conversion, the converted digital luminance signals are added to obtain a one-dimensional sum signal, and this sum signal is detected. Therefore, the automatic tracking apparatus must comprise a frame memory for temporarily storing the digital luminance signals, resulting in a large circuit arrangement. In addition, since luminance signals must be read out in units of pixels in order to obtain a sum signal, an arithmetic operation for addition is time-consuming.
In the techniques disclosed in Published Examined Japanese Patent Application No. 60-28475 and Published Unexamined Japanese Patent Application No. 61-12177, although tracking can be performed when an object to be tracked is present in a picture to be imaged, tracking cannot be performed when an object to be tracked moves outside a picture to be imaged. Especially when a telephoto lens is mounted to narrow a field, if a camera is moved from a first target to a second target present outside a picture and then moved toward the first target again, the first target may be lost.
When a camera is moved to photograph a scene by several frames and the photographed frames are combined later, i.e., when a so-called panorama photograph is to be taken, a photographer remembers specific objects in a scene and moves a camera with reference to the objects. Therefore, since taken photographs have an unnecessary overlapped portion or do not have a necessary portion, they sometimes cannot be combined well.
When an automatic tracking apparatus is arranged to change the size of a tracking field in accordance with the size of an object to be photographed, the number of photoelectric conversion elements to be used in projection is changed in accordance with the set size of the tracking field. Therefore, the level of a projection luminance signal is undesirably changed in accordance with setting of the size of the tracking field. That is, the signal level of the projection luminance signal is decreased when the tracking field is small and it is increased when the tracking field is large. Therefore, in a conventional system, when a correlation arithmetic operation is to be performed by using the obtained projection luminance signal, a method of performing the correlation arithmetic operation must be switched in accordance with the size of a tracking field to maintain the precision in a moving amount to be calculated of an object to be photographed constant.
In order to calculate the luminance of an object to be photographed by further multiplying or adding horizontal or vertical projection luminance signals, a certain correction arithmetic operations must be performed since the luminance of the object to be photographed changes in accordance with the size of the tracking field. When the size of the tracking field is large and the luminance of an object to be photographed is high, the obtained projection luminance signal may be too high.
When an automatic tracking apparatus is to be mounted on a video camera as disclosed in Published Unexamined Patent Application No. 60-253887, a video signal originally required by the video camera for image signal processing can be used in automatic tracking. In an instrument not generating a video signal such as a still camera, however, it is difficult to apply this technique to automatically track a moving object to be photographed. In addition, if a circuit for obtaining a video signal is additionally provided, the size of a circuit arrangement is increased.
When an automatic tracking apparatus is to be mounted on a still camera such as a single-lens reflex camera as disclosed in Published Unexamined Japanese Patent Application No. 64-1921, an optical system different from an optical system for film photographing must be provided to arrange an automatic tracking imaging device therein. In this single-lens reflex camera, a finder optical system is provided independently of the film photographing optical system. By moving the position of a reflecting mirror arranged behind a photographic lens, light from an object to be photographed transmitted through the photographic lens is controlled to be guided to the film photographing optical system or the finder optical system. Therefore, the automatic tracking imaging device can be most easily arranged in the finder optical system.
When the automatic tracking imaging device is arranged in the finder optical system, however, automatic tracking cannot be performed during photographing since light from an object to be photographed does not reach the finder optical system. Therefore, if an object to be photographed moves during photographing, the automatic tracking apparatus cannot recognize the position of the object to be photographed after photographing. Especially in an automatic tracking apparatus of a type which sets a specific region in a picture to be taken as a tracking field and tracks an object to be photographed in this tracking field, if an object to be photographed present in the tracking field immediately before photographing moves outside the tracking field immediately after photographing, an entirely different object to be photographed may be tracked thereafter.